Heat pump devices exist that supply a high-frequency low voltage to a compressor during a shutdown during heating in order to improve the rising speed of the air conditioner when heating is started (for example, see Patent Literature 1). A similar technique is used in a heat pump device that supplies a single-phase alternating-current voltage having a higher frequency than that at the time of a normal operation to a compressor when it is detected that the temperature of the air conditioner's surroundings becomes low (for example, see Patent Literature 2).
Moreover, in order to prevent the refrigerant stagnation phenomenon from occurring, a heat pump device exists that generates, as driving signals for a compressor motor, signals to be output with a predetermined static phase angle in the PWM output in a two-phase modulation system during the restricted energization for preheating the compressor (for example, see Patent Literature 3). A technology of stepping down, with a converter unit, an input voltage to an inverter is described in Patent Literature 4.